Can someone explain ORA-29861 error in plain english and its possible cause?

To locate broken index use:

select index_name,index_type,status,domidx_status,domidx_opstatus from user_indexes where index_type like '%DOMAIN%' and (domidx_status <> 'VALID' or domidx_opstatus <> 'VALID');

To rebuild the index use:

alter index INDEX_NAME rebuild;